The state of Oklahoma botched one execution and was forced to call off another on Tuesday when a disputed cocktail of drugs failed to kill a condemned prisoner who was left writhing on the gurney.

After the failure of a 20-minute attempt to execute him, Clayton Lockett was left to die of a heart attack in the execution chamber at the Oklahoma state penitentiary in McAlester. A lawyer said Lockett had effectively been "tortured to death".

For three minutes after the first drugs were delivered Lockett struggled violently, groaned and writhed, lifting his shoulders and head from the gurney.

Some 16 minutes after the execution began, and without Lockett being declared dead, the blinds separating the chamber from the viewing room were closed. The process was called off shortly afterwards. Lockett died 43 minutes after the first executions drugs were adminsitered.

The execution of Charles Warner, scheduled for 8pm local time, was then postponed. Both were due to have been carried out with a drug cocktail using dosages never before tried in American executions.

Lockett, 38, was convicted of the killing of 19-year-old, Stephanie Neiman, in 1999. She was shot and buried alive. Lockett was also convicted of raping her friend in the violent home invasion that lead to Neiman's death.

Warner, 46, was found guilty of raping and killing 11-month-old Adrianna Waller in 1997. He lived with the child's mother.

Death penalty states have scrambled to find new execution methods after drugs companies opposed to capital punishment, mostly based in Europe, withdrew their supplies.

Oklahoma decided to lethally inject Lockett and Warner with midazolam, a powerful sedative and anti-seizure drug that also induces amnesia, followed by vecuronium bromide and potassium chloride. Florida has used a similar method but it employed a dose of midazolam five times greater. Ohio used midazolam alongside a different drug, hydromorphone, in the January execution of Dennis McGuire, which took more than 20 minutes.

The grim outcome on Tuesday in Oklahoma appeared likely to fuel the debate over the death penalty in the US, in particular the use of these untested drugs combinations.

Madeline Cohen, an attorney for Warner, condemned the way Lockett was killed. "After weeks of Oklahoma refusing to disclose basic information about the drugs for tonight's lethal injection procedures, tonight Clayton Lockett was tortured to death," she said.

Richard Dieter, the executive director of the Death Penalty Information Center, which monitors capital punishment, said: "This could be a real turning point in the whole debate as people get disgusted by this sort of thing.

"This might lead to a halt in executions until states can prove they can do it without problems. Someone was killed tonight by incompetence," he told the Associated Press.

Before the attempted executions in Oklahoma, corrections spokesman Jerry Massie said they would probably take longer than normal because the first drug was expected to work more slowly. "Don't be surprised," Massie said.

The Guardian watched as Lockett was asked if he had final words. He said "no." He lay covered in a white sheet when the execution began at 6.23pm. At 6.30pm he was found to be still conscious.

Lockett was then pronounced unconscious at 6.33pm but his violent struggle began three minutes later. He tried to speak and was heard to say "man" at 6.39pm. An official in the execution room then lowered the blinds so viewers could no longer witness the process.

Robert Patton, the director of Oklahoma's department of corrections, said later that when doctors felt that the drugs were not having the required effect on Lockett, they discovered that a vein had ruptured. "After conferring with the warden, and unknown how much drugs went into him, it was my decision at that time to stop the execution," Patton told reporters.

Massie said that all three drugs in the cocktail used by the state were administered, but that a vein "blew" during the execution process and Lockett later suffered a heart attack. He was pronounced dead at 7.06pm, 43 minutes after the process began.

The execution of Charles Warner was postponed for 14 days.

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The double executions were scheduled after an unprecedented legal and political dispute in Oklahoma. The inmates challenged the secrecy surrounding Oklahoma's source of lethal injection drugs, winning at the state district court level, but two higher courts argued over which could grant a stay of execution.

When the state supreme court stayed their executions so that it could consider their constitutional claim, the Republican governor, Mary Fallin, declared in a controversial statement that it had no authority to grant the stay. A member of the Oklahoma House of Representatives said he would try to have the justices who wanted the stay impeached. Amid accusations of undue political pressure, the court then ruled against the prisoners and lifted the stay.

On Tuesday night Fallin said she had directed her officials to conduct an investigation. "I have asked the department of corrections to conduct a full review of Oklahoma's execution procedures to determine what happened, and why, during this evening's execution of Clayton Derrell Lockett," she said in a statement. "I have issued an executive order delaying the execution of Charles Frederick Warner for 14 days to allow for that review to be completed."

Susanna Gattoni and Seth Day, attorneys for Lockett and Warner, said Lockett's execution demonstrated the harm caused by secrecy surrounding the drugs used in the attempted executions.

"This is exactly why we fought so hard to get this information known not just for our clients but for everyone," said Gattoni. " This shouldn't be kept secret. This is unfortunately what happens."

"There will be a next step," Day said. "Whatever it is there will be a next step."

Cohen, Warner's attorney, said no executions should proceed in Oklahoma in light of Lockett's execution. "My feeling about this is there can be no more executions in Oklahoma until there is a full investigation into what went wrong, an autopsy by an independent pathologist and full transparency about this process including the drugs," Cohen said.